OJAI, California  The Matilija Dam isn't much of a dam anymore; on rainy days, it looks more like a waterfall. A pile of sediment has built up so high behind the dam that when just an inch of rain falls, water spills over in glistening cascades.

The dam's aging concrete also chokes off sediment and nutrients that could nurture the riverbanks and restore Ventura County beaches downstream. So it's got to go.

But tearing down the Southern California structure presents a costly challenge. The sheer size of its removal will make it one of the most complicated in the country, and the project will carry an expected price tag of $130 million.

"It's not just something that you can go in there and remove in a day," said Steve Evans, conservation director of the Friends of the River, which monitors dam removals across California.

Environmentalists and engineers agree the Matilija Dam has outlived its intended purpose. Officials add that demolishing the 198-foot-high dam would ultimately improve the area's ecosystem, helping restore endangered steelhead trout by allowing them to swim upstream and spawn and by allowing sand to flow downstream and restore eroded beaches.

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Jeff Pratt, director of the Ventura County Watershed Protection District, said the sediment makes the reservoir all but useless. The protection district is leading the removal effort.

Plans to remove the Matilija Dam meet state coastal protection requirements. The dam, built in 1947, was created as a means of providing flood control to a handful of small downstream communities and recharging groundwater supplies used by farmers in the sparsely populated Ojai Valley.

Now, the dam cradles mostly rocks and pebbles and is clogged with 6 million cubic yards of sediment.

"For a long time we looked at the benefits of water development  dams, levees, and such  and ignored the costs," said Daniel McCool, a University of Utah political science professor who studies environmental issues. "Now the things that were damaged by the levees and dams are the things that we value."

Part of the Matilija is already gone. Workers took out slabs built with substandard materials, reducing the dam's height to 165 feet in the middle. As the rest is removed, the watershed district will take steps to avoid flooding along the 16 miles of the Ventura River that leads to the Pacific Ocean.

Nationally, at least 145 aging dams have been torn down in the last five years for a host of reasons, including safety.

Among other restoration projects:

Environmentalists want to restore the Louisiana coastline, harmed by dams and channels on the Mississippi. In central Florida, ecologists are removing human-made channels from the Kissimmee River that destroyed wetlands and wildlife.

And on Washington's Olympic Peninsula, planning is under way to dismantle the 108-foot-tall Elwha Dam and the 210-foot-tall Glines Canyon Dam as part of a $182 million plan to restore the Elwha River, reopening 70 miles of salmon and steelhead habitat.

The Matilija's removal won't begin until at least 2007. A proposed plan has been presented for public comment, and funding will need to come mostly from Congress.

The riverbank ranges from lushly shaded to dry and craggy where the river goes underground.

To help what fish remain, engineers designed a series of small pools leading to a holding pond where the trout would be picked up and driven upstream by truck. But rocks carried over the dam by water broke the fish ladder.

The leading proposal for removing the dam calls for gradually pumping 2 million cubic yards of mud to the flood plain downstream. After temporarily stabilizing the rest of the sediment, crews would break the dam down a section at a time, said Pam Lindsey, a watershed district ecologist.

Among the land that could be partially flooded is the riverside estate of Brooks Greene-Barton, a former real estate broker. Greene-Barton supports the dam's removal, even though he and his wife were in escrow to sell the 10-acre property when the prospective buyer backed out because of flooding concerns.

Instead of filing suit, Greene-Barton hopes to lease his land to the district. As an added benefit, he would retain the lush riverbank where his two Labrador retrievers can still play.